Process and composition for chromium plating



United States Patent 3,108,933 PROCESS AND COMPOSITION FOR CHROMIUM PLATING Andy Albert Johnson, Oak Park, Mich., assignor, by mesne assignments, to M. & T. Chemicals, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Feb. 28, 1961, Ser. No. 92,158 15 Claims. (Cl. 204-51) The present invention relates to a process for electrodepositing crack-free chromium, and more particularly to an improved process for electrodepositing bright crackfree chromium.

Bright chromium as electrodeposited from chromic acid baths in the conventional process for decorative plate usually cracks when the deposit reaches a thickness of about 0.00002 inch. The tendency to crack increases as the thickness increases. Several processes have recently been developed for electrodepositing bright crack-free chromium to a thickness of at least 0.00005 inch. Thick crack-free chromium electrodeposits have superior corrosion resistance.

It is an object of this invention to provide an improved process for electroplating thick crack-free chromium. It is another object of this invention to provide baths from which thick crack-free chromium may be deposited. Still another object of the invention is to provide novel compositions of matter useful for making up and maintaining chromic acid baths for the electrodeposition of crackfree chromium. The present invention also contemplates an improved process for obtaining thicker deposits of bright crack-free chromium.

Processes for the electrodeposition of industrial crackfree chromium are disclosed in US. Patent Nos. 2,686,- 756, 2,787,588, and 2,787,589. New processes have recently been developed for the electrodeposition of bright crack-free chromium from sulfate catalyzed baths as well as baths catalyzed with sulfate plus a complex fluoride, as disclosed in US. Patent Nos. 2,916,424 and 2,952,590, and in the Safranek et al. article in the May 1960 issue of Plating, vol. 47 (pages l3519). I discovered that when electrodepositing chromium, using the conditions specified for each of these processes, and using the specified baths which had been modified only by the addition of lithium, a greater thickness of chromium was electrodeposited before cracking occurred. By utilizing baths containing lithium in the aforesaid processes, it is possible to broaden the conditions under which bright crack-free chromium may be electrodeposited to a minimum thickness of at least 0.00005 inch. It is known that lower plating temperatures tend to decrease the thickness at which cracking occurs. Higher concentrations of catalyst in the bath also tend to decrease the thickness at which cracking occurs. Lithium additions to the bath tend to counteract the above tendencies. Where suitable conditions have been determined for obtaining a specified thickness of bright crack-free chromium in a lithium-free bath, it is possible to obtain the same thickness using lower temperatures and/ or lower ratios (of CrO to catalyst ion) with a lithium-containing bath. In etfect, processes utilizing lithium-containing baths under specified conditions to obtain a desired thickness of crack-free chromium are the equivalent of corresponding processes utilizing lithium-free baths at higher temperatures and/or higher ratios.

Relatively small additions of lithium to the bath increase the thickness which is plated before cracking occurs. Amounts as small as 1g./l. (gram per liter) lithium in the bath yield substantial improvement in this respect. It is preferred that baths contain a minimum of 2 g./l. and preferably 4 g./l. of lithium to obtain substantial increases in the thickness of crack-free electrodeposit.

Substantially larger amounts of lithium in the bath may be tolerated and are even useful in increasing the thickness of plate before cracking occurs. The maximum amount of lithium in the bath is governed by the percent neutralization of the bath. For commercial practice, it is not contemplated that baths neutralized more than about to the dichromate end-point will be used. For electrodeposition of bright crack-free chromium, it is prefered that baths have compositions within the following ranges: CrO -between 100g./l. and 500 g./l. and preferably between 200 g./l. and 400 g./l.; sulfate ion-- between 0.25 g./l. and 3.5 g./l. and preferably between 1 g./l. and 2.5 gl./l.; complex fluoride ion-between 0.5 g./l. and 5 g./l. and preferably between 1 g./l. and 2.5 g./l.; and having a ratio between 70:1 to 150:1 and preferably between :1 to 130:1.

Chromium plating baths are frequently designated as chromic acid baths. Herein the chromic acid content of the bath is referred to as CrO (more accurately designated chromic anhydride). The bath may be made up by supplying CrO in the form of chromic anhydride or in the form of soluble CrO -containing compounds containing cations which do not adversely affect the bath characteristics. Such compounds include the chromates, dichromates, and polychromates of potassium, sodium, magnesium, and calcium. The CrO may also be added in the form of chromic acid and/or dichromic acid in solution. The term CrO is used herein to refer to the compound per se and to the CrO -containing compounds as defined hereinabove, in solution and in solid form. Sulfate ions may be added in the form of sulfuric acid, or as soluble sulfate salt(s) with a cation that does not adversely affect bath characteristics. Similarly, the fluoaluminate, fluotitanate, fluozirconate, and/or silicofluoride anions may be added to the baths as the soluble salts or they may be formed in situ by the introduction into the chromic acid bath of the elements and/0r compounds which in the bath environment react to form the desired ion. Lithium may be added to the bath in the form of the metal which dissolves as the cation, or it may be added in the form of a soluble compound such as lithium carbonate, lithium bicarbonate, lithium chromate, lithium dichromate, lithium hydroxide, and lithium oxide. It is preferably added in the form of the chromate and/ or dichromate. The term lithium" is used herein to define the element and ion in any of the aformentioned equivalents in solution and in solid form. The processes of electrodepositing of bright crack-free chromium require careful control of the bath components and particularly the CrO the catalyst anion, and the ratio between the two. My process also requires control of the lithium concentration. To accomplish this control it is preferred that the chromium plating baths be made up and maintained by the addition of compositions of matter which contain all the necessary bath components, i.e. the CrO the catalyst ions, and the lithium cation. Compositions of matter for making up and maintaining the baths preferably contain, in addition to the CrO and the sulfate-supplying material and/ or the complex fluoride-supplying materials, between about 0.2% and 5% of lithium cation. The compositions for the mixed catalyst baths contain between 70 parts and parts of CrO between 0.5 part and 1.5 parts of sulfate anion, between 0.5 part and 5 parts of complex fluoride anions, and between 0.2 part and 5 parts (preferably 0.5 part and 2 parts) of lithium. These compositions may also contain some water and ancillary additives such as foam suppressants, etc. Using the above compositions of matter which are in solid form, the bath is made up with water to the desired chromic acid concentration for plating. The same compositions of matter are useful for maintaining the bath.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative examples are given:

A standard Hull Cell or its equivalent was used to determine the thickness to which chromium plate can be electrodeposited without cracking. The Hull Cell is a container with an anode perpendicular to the sides and an inclined cathode. The current density on the cathode varies inversely with the distance from the near end where the current density is highest and the electrodeposit is thickest. The chromium plated cathode was examined for cracks and for brightness, and a thickness measurement made at the region where the crack pattern begins. Bright crack-free chromium was electrodeposited from all the baths to the specified thickness, noted in the table.

Fluoride Complex Thickness to Crackint, 10' inches Anion g./l.

The examples illustrate the advantages to be gained by the addition of lithium to the baths. In the examples, a comparison is made of the thickness to which chromium is electrocleposited before cracking occurs, with and without lithium addition, and/ or a comparison showing the superior results obtained by a larger lithium addition over a smaller one. The presence in the bath of cer tain other materials such as chlorides are known to promote cracking. Such materials must be kept out of the bath or held to a minimum concentration.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it

is to be understood that the invention includes all such modifications and variations as come within the scope of the appended claims.

I claim:

1. In a process for electrodepositing crack-free chromium from an aqueous bath at a temperature of at least 115 F. and containing between 100 -g./l. and 500 g./l. of CrO at least one catalyst ion selected from the class consisting of sulfate, silicofluoride, fluoaluminate, fluotitanate, and fluozirconate, in an amount between 0.5 g./l. and 3.5 g./-l., and having a ratio of CrO to total catalyst ions of between about 85 :1 to 150:1, the improvement which comprises adding to said bath sufiicient lithium so that the bath contains at least 1 g./l. of lithium ion.

2. In a process for elect-rodepositing bright crack-free chromium from an aqueous bath at a temperature of at least 115 F. and containing between 100 g./l. and 500 g./l. of CrO between 0.25 g./l. and 3.5 g./l. of sulfate ion, between 0.5 g./l. and 5 g./l. of a complex ion selected from the class consisting of silicotluoride, fluoaluminate, fluotitanate, and fiuozirconate, the ratio of CrO to the total of the sulfate and complex ions being between 85:1 and 150:1, the improvement which comprises adding to said bath suflicient lithium so that the bath contains at least 1 g./l. of lithium ion.

3. In a process for electrodepositing bright crack-free chromium from. an aqueous bath at a temperature of at least 115 F. and containing between about 200 g./l. and 400 g./l. of CrO between 1 g./l. and 2.5 g./l. of sulfate ion, between 1 -g./l. and 2.5 g./l. of a complex io-n selected from the class consisting of silicofluoride, fluoaluminate, fluotitanate, and fluozirconate, the ratio of Cr0 to the total of the sulfate and complex ions being between about :1 and 130:1, the improvement which comprises adding to said bath sufiicient lithium so that the bath contains at least 2 g./l. of lithium ion.

4. The process of claim 3 wherein the bath contains at least 4 g./l. of lithium. ion.

5. In a process for electrodcpositing bright crack-free chromium from an aqueous bath at a temperature of at least 115 F. and containing between g./l. and 500 g./l. of CrO between 0.25 g./l. and 3.5 g./l. of sulfate ion, and between 0.5 g./l. and 5 g./l. of silicofluoride ion, and having a ratio of C10 to the total of the sulfate and silicofiuoride ions between 85:1 to 150:1, the improvement which comprises adding to said bath suflicient lithium so that the bath contains at least 1 -g./l. of lithium ion.

6. In a process for electrodepositing bright crack-free chromium from an aqueous bath at a temperature of at least F. and containing between 200 g./l. and 400 g./l. of CrO between 1 g./l. and 2.5 g./l. of sulfate ion, and between 1 .g./l. and 2.5 g./ l. of silicofiuoride ion, and having a ratio of CrO to the total of the sulfate and silicofiuoride ions between about 85 :1 to :1, the improvement which comprises adding to said bath suificient lithium so that the bath contains at least 2 g./l. of lithium ion.

7. In a process for electrodepositing bright crack-free chromium from an aqueous bath at a temperature of at least 115 F. and containing between 100 g./l. and 500 g./l. of CrO between 0.25 g./l. and 3.5 g./l. of sulfate ion, and between 0.5 g./l. and 5 g./l. of fluoalurninate ion, and having a ratio of CrO to the total of the sulfate and fluoaluminate ions between 85:1 to :1, the improvement which comprises adding to said bath sufiicient lithium so that the bath contains at least 1 g./l. of lithium ion.

8. In a process for electrodepositing bright crack-free chromium from an aqueous bath at a temperature of at least 115 F. and containing between 100 g./l. and 500 g./l. of CrO between 0.25 g./l. and 3.5 g./l. of sulfate ion, and between 0.5 g./l. and 5 g./l. of fluotitanate ion, and having a ratio of CrO to the total of the sulfate and fluotitanate ions between 85:1 to 150:1, the improvement which comprises adding to said bath sufiicient lithium so that the bath contains at least 1 g./l. of lithium ion.

9. In a process for electrodepositing bright crack-tree chromium from an aqueousbath at a temperature of at least 115 F. and containing between 100 g./l. and 500 g./l. of CrO between 0.25 g./1. and 3.5 g./l. of sulfate ion, and between 0.5 g./l. and 5 g./1. of fiuozirconate ion, and having a ratio of G0,, to the total of the sulfate and fluozirco-nate ions between 85 :1 to 150:1, the improvement which comprises adding to said bath sufiicient lithium so that the bath contains at least 1 g./l. of lithium ion.

10. In a process for electrodepositing bright crack-free chromium from an aqueous bath at a temperature of at least 115 F. and containing between 200 g./l. and 400 g./l. of OrO between 1 g./l. and 2.5 g./l. of sulfate ion, and between 1 g./l. and 2.5 g./l. of fluoaluminate ion, and having a ratio of CrO to the total of the sulfate and fiuoaluminate ions between about 85:1 to 130:1, the improvement which comprises adding to said bath sufficient lithium so that the bath contains at least 2 g./l. of lithium ion.

11. A composition of matter for making up and mainmining aqueous baths for use in an improved process for eiectrodepositing crack-free chromium, said composition of matter comprising between 70 and 95 parts of CrO between 0.5 and 1.5 parts of sulfate, between 0.5 and 5 parts of at least one complex fluoride selected from the class consisting of silicofiuoride, fiuoaluminate, fluotitanate, and fluozirconate; and between 0.2 and 5 parts of lithium.

12. A composition of matter as defined in claim 11, containing between 0.5 and 2 par-ts of lithium.

13. A composition of matter for making up and maintaining aqueous baths suitable for use in an improved process for electrodepositing bright crack-free chromium, said composition of matter comprising between 70 and 15. The process of claim 1 wherein the catalyst ion is sulfate.

References Cited in the file of this patent UNITED STATES PATENTS 2,916,424 Stnreck et al Dec. 8, 1959 FOREIGN PATENTS 299,395 Great Britain Ian. 24, 1930 

1. IN A PROCESS FOR ELECTRODEPOSITING CRACK-FREE CHROMIUM FROM AN AQUEOUS BATH AT A TEMPERATURE OF AT LEAST 115*F. AND CONTAINING BETWEEN 100 G./L. AND 500 G./L. OF CRO3, AT LEAST ONE CATALYST ION SELECTED FROM THE CLASS CONSISTING OF SULFATE, SILICOFLUORIDE, FLUOALUMINATE, FLUOTITANATE, AND FLUOZZIRCONATE, IN AN AMOUNT BETWEEN 0.5 G./L. AND 3.5 G./L., AND HAVING A RATIO OF CRO3 TO TOTAL CATALYST IONS OF BETWEEN ABOUT 85:1 TO 150:1, THE IMPROVEMENT WHICH COMPRISES ADDING TO SAID BATH SUFFICIENT LITHIUM SO THAT THE BATH CONTAINS AT LEAST 1 G./L. OF LITHIUM ION. 